Research Article
Nanorod diameter modulated osteogenic activity of hierarchical micropore/nanorod-patterned coatings via a Wnt/β-catenin pathway

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Abstract

Hierarchical micropore/nanorod-patterned strontium doped hydroxyapatite (Ca9Sr1(PO4)6(OH)2, Sr1-HA) structures (MNRs) with different nanorod diameters of about 30, 70 and 150 nm were coated on titanium, to investigate the effect of nanorod diameter on osteogenesis and the involved mechanism. Compared to micropore/nanogranule-patterned Sr1-HA coating (MNG), MNRs gave rise to dramatically enhanced in vitro mesenchymal stem cell functions including osteogenic differentiation in the absence of osteogenic supplements and in vivo osseointegration related to the nanorod diameter with about 70 nm displaying the best effects. MNRs activated the cellular Wnt/β-catenin pathway by increasing the expression of Wnt3a and LRP6 and decreasing the expression of Wnt/β-catenin pathway antagonists (sFRP1, sFRP2, Dkk1 and Dkk2). The exogenous Wnt3a significantly enhanced the β-catenin signaling activation and cell differentiation on MNG, and the exogenous Dkk1 attenuated the enhancing effect of MNRs on them. The data demonstrate that MNRs favor osseointegration via a Wnt/β-catenin pathway.

Graphical Abstract

Hierarchical micropore/nanorod-patterned strontium doped hydroxyapatite (Ca9Sr1(PO4)6(OH)2, Sr1-HA) structures (MNRs) with different nanorod diameters of about 30, 70 and 150 nm were coated on Titanium. Compared to micropore/nanogranule-patterned Sr1-HA coating (MNG), MNRs dramatically enhanced in vitro mesenchymal stem cell adhesion, attachment, spreading, proliferation and osteogenic differentiation as well as in vivo contact osteogenesis and integration related to the nanorod diameter with about 70 nm displaying the best effects, which is attributed to the focal adhesion formation favoring effect and the related mechanotransduction. The Wnt/β-catenin pathway is involved in the MNRs induced indirect mechanotransduction.

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Section snippets

Material preparation and characterization

The formation of MNG was described in detail in our previous works.5, 7 A two-step method was performed to fabricate Sr1-HA MNRs with different nanorod diameters of 30, 70 and 150 nm in average, designated as MNR-D30, MNR-D70, and MNR-D150, where the Arabic numbers indicate the mean nanorod diameter. Firstly, the Ti disks (Ф15×2 mm) were treated by micro-arc oxidation (MAO) in an aqueous electrolyte containing 0.167 M CA, 0.033 M SA and 0.020 M β-GP with an applied positive pulse voltage of 380

Structural characterization of the coatings

As shown in Figure 1, A, MNG and MNRs all show typical microporous MAO surface feature, with micropores of an average diameter of 3-4 μm distributing homogeneously. However, different from the nanogranulated 2D topography on MNG, MNRs present nanorod-shaped 3D topographies with a similar interrod spacing of about 70 nm but different nanorod diameters of 153.82 ± 7.82, 67.23 ± 6.25 and 31.75 ± 4.19 nm (Table S2), denoted as MNR-D150, MNR-D70 and MNR-D30, respectively. The XRD pattern in Figure 1

Discussion

The potential surface properties of a biomaterial influencing the protein adsorption ability and consequent biological performance include surface chemistry, surface roughness, surface wettability and surface structure.31 All the coatings in this study have the same top-layer component of Sr1-HA that shows good cytocompatibility to support the growth and osteogenic differentiation of MSCs.7, 32 The coatings also have similar microporous feature and submicroscale roughness. MNRs show

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    Conflict of Interest: Authors report no conflict of interest.

    Sources of Support: National Natural Science Foundation of China (51631007, 81401527 and 51371137), China Postdoctoral Science Foundation (2014M560771 and 2016T90912), Postdoctoral Research Program of Shaanxi Province, State Key Laboratory for Mechanical Behavior of Materials (20171904) and Baoji University of Arts and Sciences Key Research (ZK15042).

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    Contributed equally to this work.

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